1. Field of the Invention
Apparatuses consistent with the present invention relate to a reflection-type bandpass filter for use in ultra-wideband (UWB) radio data communications. By using this reflection-type bandpass filter for UWB, the spectrum mask established by the Federal Communications Commission (FCC) can be satisfied.
2. Description of the Related Art
The following documents describe the related art for the current invention:
[Document 1] U.S. Pat. No. 2,411,555 Specification
[Document 2] Japanese Unexamined Patent Application, First Publication No. S56-64501
[Document 3] Japanese Unexamined Patent Application, First Publication No. H9-172318
[Document 4] Japanese Unexamined Patent Application, First Publication No. H9-232820
[Document 5] Japanese Unexamined Patent Application, First Publication No. H10-65402
[Document 6] Japanese Unexamined Patent Application, First Publication No. H10-242746
[Document 7] Japanese Unexamined Patent Application, First Publication No. 2000-4108
[Document 8] Japanese Unexamined Patent Application, First Publication No. 2000-101301
[Document 9] Japanese Unexamined Patent Application, First Publication No. 2002-43810
[Document 10] A. V. Oppenheim and R. W. Schafer, “Discrete-time signal processing,” pp. 465-478, Prenticehall, 1998
[Document 11] G-B. Xiao, K. Yashiro, N, Guan, and S. Ohokawa, “An effective method for designing non-uniformly coupled transmission-line filters,” IEEE Trans. Microwave Theory tech., vol. 49, pp. 1027-1031, June 2001.
[Document 12] C-Y. Chen and C-Y. Hsu, “Design of a UWB low insertion loss bandpass filter with spurious response suppression,” Microwave J., pp. 112-116, February 2006
In bandpass filters of the related art, the stop band rejection (difference between the reflectivity in the pass band and reflectivity in the stop band) was not set at an adequately large value in the design stage. Thus, these filters may not satisfy the FCC regulations because of manufacturing errors and the like.
For example, if a microstrip line as in FIG. 1 having a distribution as shown in FIG. 2, which is a distribution in the lengthwise direction of width of a microstrip line is used (when substrate with thickness h=0.635 mm, relative dielectric constant ∈r=10.2 is used), as shown in FIG. 3, the absolute value of the difference between the reflectivity when the frequency f is in the region 3.4 GHz≦f≦10.3 GHz, and the reflectivity when f<3.1 GHz or f>10.6 GHz, that is, the stop band rejection, becomes 10 dB approximately. Therefore, because of a small manufacturing error, the stop band rejection may drop below 10 dB. Also, as shown in FIG. 4, the variation of the group delay frequency characteristics is large near the transition frequency.
In Document 12, a bandpass filter provided with a dual mode-type microstrip is reported as wide-band bandpass filter for UWB. However, the pass band of the bandpass filter disclosed in Document 12 is between 3 GHz and 5.5 GHz approximately. Compared to the band prescribed by the FCC, the pass band is narrow, and it does not cover the entire region of the UWB. The design method for the bandpass filter disclosed in Document 12 is complicated, and difficult to realize.